Never actually watched on (you are entitled to watch if you want) but it can’t be that thorough as the maximum charge is set by law at £54.85.How long does that MOT thing take that y'all have to do? It's pretty thorough?
If you fail then you can either leave the car at the same shop to be fixed, or take it elsewhere but then have to pay the fee again. Of course there are shops who make sure they find some work needing doing every time - but that’s a bad strategy as customers vote with their feet. Conversely the one I use very rarely finds more than 'advisories' i.e. Things which are OK but might not be by next year.
Where you will definitely have trouble is if you fail the emissions test. But recently they introduced a rule that warning lights must not be showing and equipment fitted must work... so lots of people remove things for the MOT.